eventstream.go

/****************************************************
Copyright 2018 The ont-eventbus Authors

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

    http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.
*****************************************************/

/***************************************************
Copyright 2016 https://github.com/AsynkronIT/protoactor-go

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

    http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.
*****************************************************/
package eventstream

import (
	"sync"
)

// Predicate is a function used to filter messages before being forwarded to a subscriber
type Predicate func(evt interface{}) bool

var es = &EventStream{}

func Subscribe(fn func(evt interface{})) *Subscription {
	return es.Subscribe(fn)
}

func Unsubscribe(sub *Subscription) {
	es.Unsubscribe(sub)
}

func Publish(event interface{}) {
	es.Publish(event)
}

type EventStream struct {
	sync.RWMutex
	subscriptions []*Subscription
}

func (es *EventStream) Subscribe(fn func(evt interface{})) *Subscription {
	es.Lock()
	sub := &Subscription{
		es: es,
		i:  len(es.subscriptions),
		fn: fn,
	}
	es.subscriptions = append(es.subscriptions, sub)
	es.Unlock()
	return sub
}

func (ps *EventStream) Unsubscribe(sub *Subscription) {
	if sub.i == -1 {
		return
	}

	ps.Lock()
	i := sub.i
	l := len(ps.subscriptions) - 1

	ps.subscriptions[i] = ps.subscriptions[l]
	ps.subscriptions[i].i = i
	ps.subscriptions[l] = nil
	ps.subscriptions = ps.subscriptions[:l]
	sub.i = -1

	// TODO(SGC): implement resizing
	if len(ps.subscriptions) == 0 {
		ps.subscriptions = nil
	}

	ps.Unlock()
}

func (ps *EventStream) Publish(evt interface{}) {
	ps.RLock()
	defer ps.RUnlock()

	for _, s := range ps.subscriptions {
		if s.p == nil || s.p(evt) {
			s.fn(evt)
		}
	}
}

// Subscription is returned from the Subscribe function.
//
// This value and can be passed to Unsubscribe when the observer is no longer interested in receiving messages
type Subscription struct {
	es *EventStream
	i  int
	fn func(event interface{})
	p  Predicate
}

// WithPredicate sets a predicate to filter messages passed to the subscriber
func (s *Subscription) WithPredicate(p Predicate) *Subscription {
	s.es.Lock()
	s.p = p
	s.es.Unlock()
	return s
}